Effects of Different Ventilation Ratios on γ-Polyglutamic Acid Production by Bacillus licheniformis Fermentation Based on Metabolomics and Transcriptomics

To investigate the effects of different ventilation ratios on γ-polyglutamic acid (PGA) production by microbial fermentation, using Bacillus licheniformis BYBC1.21056 as the starting strain, the effects of different ventilation ratios (1.0:0.7, 1.0:1.0, 1.0:1.3, 1.0:1.6, 1.0:1.9) on intracellular metabolism and gene expression of strain BYBC1.21056 were analyzed by combining metabolomics and transcriptomics. The results showed that the high ventilation ratio (1.0:1.9) most favored γ-PGA synthesis, achieving a yield of 105.54 g/L, demonstrating that ventilation was a key factor influencing fermentation. Metabolomic analysis results showed that a total of 916, 1 164, and 1 081 significantly different metabolites were identified between the experimental group and the control group, respectively, after fermentation 24, 48 and 72 h. Among them, the unique significantly different metabolites were 293, 563 and 412, respectively. Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis results indicated that, compared with the control group, the significantly upregulated metabolic pathways for significantly different metabolites were tryptophan metabolism, nucleotide metabolism, glycerophospholipid metabolism, purine metabolism, nicotinic acid and nicotinamide metabolism, etc. in the experimental group after fermentation 24, 48, and 72 h. Transcriptome analysis results revealed that there were 882 upregulated genes and 1 025 downregulated genes at 24 h, 373 upregulated genes and 506 downregulated genes at 48 h, and 461 upregulated genes and 394 downregulated genes at 72 h. The results showed that increasing the ventilation ratio altered gene expression in B. licheniformis, thereby regulating its metabolism and affecting γ-PGA synthesis.